Objection 1. It seems that covetousness is not opposed to liberality. For Chrysostom, commenting on Matthew 5:6, "Blessed are they that hunger and thirst after justice," says, (Hom. xv in Matth.) that there are two kinds of justice, one general, and the other special, to which covetousness is opposed: and the Philosopher says the same (Ethic. v, 2). Therefore covetousness is not opposed to liberality.
Objection 2. Further, the sin of covetousness consists in a man's exceeding the measure in the things he possesses. But this measure is appointed by justice. Therefore covetousness is directly opposed to justice and not to liberality.
Objection 3. Further, liberality is a virtue that observes the mean between two contrary vices, as the Philosopher states (Ethic. i, 7; iv, 1). But covetousness has no contrary and opposite sin, according to the Philosopher (Ethic. v, 1,2). Therefore covetousness is not opposed to liberality.
On the contrary, It is written (Ecclesiastes 5:9): "A covetous man shall not be satisfied with money, and he that loveth riches shall have no fruits from them." Now not to be satisfied with money and to love it inordinately are opposed to liberality, which observes the mean in the desire of riches. Therefore covetousness is opposed to liberality.
I answer that, Covetousness denotes immoderation with regard to riches in two ways. First, immediately in respect of the acquisition and keeping of riches. On this way a man obtains money beyond his due, by stealing or retaining another's property. This is opposed to justice, and in this sense covetousness is mentioned (Ezekiel 22:27): "Her princes in the midst of her are like wolves ravening the prey to shed blood... and to run after gains through covetousness." Secondly, it denotes immoderation in the interior affections for riches; for instance, when a man loves or desires riches too much, or takes too much pleasure in them, even if he be unwilling to steal. On this way covetousness is opposed to liberality, which moderates these affections, as stated above (117, 2, ad 3, 3, ad 3, 6). On this sense covetousness is spoken of (2 Corinthians 9:5): "That they would... prepare this blessing before promised, to be ready, so as a blessing, not as covetousness," where a gloss observes: "Lest they should regret what they had given, and give but little."
Reply to Objection 1. Chrysostom and the Philosopher are speaking of covetousness in the first sense: covetousness in the second sense is called illiberality [aneleutheria] by the Philosopher.
Reply to Objection 2. It belongs properly to justice to appoint the measure in the acquisition and keeping of riches from the point of view of legal due, so that a man should neither take nor retain another's property. But liberality appoints the measure of reason, principally in the interior affections, and consequently in the exterior taking and keeping of money, and in the spending of the same, in so far as these proceed from the interior affection, looking at the matter from the point of view not of the legal but of the moral debt, which latter depends on the rule of reason.
Reply to Objection 3. Covetousness as opposed to justice has no opposite vice: since it consists in having more than one ought according to justice, the contrary of which is to have less than one ought, and this is not a sin but a punishment. But covetousness as opposed to liberality has the vice of prodigality opposed to it.
Source: Summa Theologica (New Advent)